As wireless communications technology continues to advance, wireless power transmitting/receiving technology which allows power to be wirelessly transmitted to electronic devices has started to receive more attention. Wireless power transmitting/receiving technology may be diversely applied not only to battery charging of portable terminals, but also to the delivery of power to home appliances, electric cars, or subways.
Wireless power transmitting/receiving technology uses the fundamental principle of magnetic induction. In this case, in order to increase the efficiency of power transmission and reception, it is necessary to align a transmitting coil of the wireless power transmitting apparatus and a receiving coil of the wireless power receiving apparatus. If there is a foreign object (FO) between the wireless power transmitting apparatus and the wireless power receiving apparatus, not only does the efficiency of power transmission and reception become low, but also there may be the concern of getting burned or a fire breaking out due to the rise in temperature around an area where the wireless power transmitting apparatus comes into contact with the foreign object.
Representative examples of these foreign objects are a disc-shaped coin, a ring-shaped accessory, a foil-type thin metal plate, and the like, and especially a foreign object which is formed of steel or aluminum causes problems.
In general, the wireless power transmitting apparatus periodically sends a ping signal for determining whether there is a wireless power receiving apparatus, and in the case that there is a wireless power receiving apparatus, after an identification and configuration process, the wireless power transmitting apparatus transfers power to the wireless power receiving apparatus.
During the transfer of power, the wireless power transmitting apparatus calculates the amount of power loss due to a foreign object, and if an amount of the power loss is greater than a predetermined level, it is assumed that there is a foreign object and the transfer of power may be stopped. Or, during the transfer of power, the wireless power transmitting apparatus detects a temperature of a surface, and if the temperature rises above a predetermined level, it is assumed that there is a foreign object and the transfer of power may be stopped.
When using this method, however, a timing of detecting a foreign object is late such that heat has already been generated, thus the power transfer stops when the charging efficiency becomes low. In addition, when receiving power information is not obtained from the wireless power transmitting apparatus, there is a problem in that power is continuously transmitted in a state of low charging efficiency. Furthermore, even when the wireless power receiving apparatus is not present, the wireless power transmitting apparatus becomes heated due to the ping signals which are continuously sent.